The stability, dynamics and energetics of an auroral shear layer are considered in the framework of incompressible, one-fluid magnetohydrodynamics, under conditions where current flow through the system is limited by the finite Pedersen conductivity and an enhanced field-aligned resistivity. The model includes a magnetospheric region where currents resulting from polarization electric fields and viscous forces are important, an ionospheric substrate of uniform conductivity, and a force-free acceleration region, characterized by a linear current-voltage relation and located at an intermediate altitude between the magnetospheric viscous/polarization layer and the ionosphere. It is assumed that the Alfv¿n wave transit time across the viscous/polarization layer is small compared with the eddy time. Neutral stability of the model system is determined for a class of one-dimensional equilibria in which a specified current distribution at the upper boundary of the viscous/polarization layer produces a potential structure with convergent, localized reversals in the transverse (E¿B) electric field. The calculated neutral curves depend on three nondimensional parameters related to the intensity of the imposed field-aligned current, the shear layer scale size, and the ratio of resistive to viscous drag at equilibrium.

Numerical simulations of unstable configurations show that (1) two-dimensional quasi-steady rotational states arise when the equilibrium is weakly unstable; (2) eddy shedding turbulent states can arise when the equilibrium is strongly unstable; and (3) the flow kinetic energy and energy input/dissipation rates in the model system are reduced as a consequence of the instability. Power spectral densities for the electric and magnetic fields are also evaluated along sample ''satellite'' cuts through the shear layer. An application to postnoon auroral forms confirms the tendency for 2D rotational motion and periodic bright spots, although the observed intensity of the upward field-aligned current suggests that the effective resistivity of the system is not sufficient to suppress inductive fields in the vortex dynamics. ¿1991 American Geophysical Union